Turbochargers are widely used on internal combustion engines, and in the past have been particularly used with large diesel engines, especially for highway trucks and marine applications. Compressor impeller wheels are found in both superchargers, which derive their power directly from the crankshaft of the engine, and turbochargers, which are driven by the engine exhaust gases.
More recently, in addition to use in connection with large diesel engines, turbochargers have become popular for use in connection with smaller, passenger car power plants. The use of a turbocharger in passenger car applications permits selection of a power plant that develops the same amount of horsepower from a smaller, lower mass engine. Using a lower mass engine has the desired effect of decreasing the overall weight of the car, increasing sporty performance, and enhancing fuel economy. Moreover, use of a turbocharger permits more complete combustion of the fuel delivered to the engine, thereby reducing the hydrocarbon emissions of the engine, which contributes to the highly desirable goal of a cleaner environment.
The design and function of turbochargers are described in detail in the prior art, for example, U.S. Pat. Nos. 4,705,463, 5,399,064, and 6,164,931, the disclosures of which are incorporated herein by reference.
Turbocharger units typically include a turbine operatively connected to the engine exhaust gas manifold, a compressor operatively connected to the engine air intake manifold, and a shaft connecting the turbine and compressor so that rotation of the turbine wheel causes rotation of the compressor impeller. The turbine is driven to rotate by the exhaust gas flowing in the exhaust manifold. The compressor impeller is driven to rotate by the turbine, and as it rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the engine cylinders.
Turbocharger compressors consist of three fundamental components: compressor wheel, diffuser, and housing. The compressors work by drawing air in axially, accelerating the air to a high velocity through the rotational speed of the wheel, and expelling the air in a radial direction. The diffuser slows down the high-velocity air, which in exchange increases the pressure and the temperature. The diffuser is formed by the compressor backplate and a part of the volute housing, which in turn collects the air and slows it down before it reaches the compressor exit.
The blades of a compressor wheel have a highly complex shape, for (a) drawing air in axially, (b) accelerating it centrifugally, and (c) discharging air radially outward at an elevated pressure into the volute-shaped chamber of a compressor housing. In order to accomplish these three distinct functions with maximum efficiency and minimum turbulence, the blades can be said to have three separate regions.
First, the leading edge of the blade can be described as a sharp pitch helix, adapted for scooping air in and moving air axially. Considering only the leading edge of the blade, the cantilevered or outboard tip travels faster (MPS) than the part closest to the hub, and is generally provided with an even greater pitch angle than the part closest to the hub. Thus, the angle of attack of the leading edge of the blade undergoes a twist from lower pitch near the hub to a higher pitch at the outer tip of the leading edge. Further, the leading edge of the blade generally is bowed, and is not planar. Further yet, the leading edge of the blade generally has a “dip” near the hub and a “rise” or convexity along the outer third of the blade tip. These design features are all designed to enhance the function of drawing air in axially.
Next, in the second region of the blades, the blades are curved in a manner to change the direction of the airflow from axial to radial, and at the same time to rapidly spin the air centrifugally and accelerate the air to a high velocity, so that when diffused in a volute chamber after leaving the impeller, the energy is recovered in the form of increased pressure. Air is trapped in airflow channels defined between the blades, as well as between the inner wall of the compressor wheel housing and the radially enlarged disc-like portion of the hub which defines a floor space, the housing-floor spacing narrowing in the direction of air flow.
Finally, in the third region, the blades terminate in a trailing edge, which is designed for propelling air radially out of the compressor wheel. The design of this blade trailing edge is generally complex, provided with (a) a pitch, (b) an angle offset from radial, and/or (c) a back taper or back sweep (which, together with the forward sweep at the leading edge, provides the blade with an overall “S” shape). Air expelled in this way has not only high flow, but also high pressure.
The operating behavior of a compressor within a turbocharger may be graphically illustrated by a “compressor map” associated with the turbocharger in which the pressure ratio (compression outlet pressure divided by the inlet pressure) is plotted on the vertical axis and the flow is plotted on the horizontal axis. In general, the operating behavior of a compressor wheel is limited on the left side of the compressor map by a “surge line” and on the right side of the compressor map by a “choke line.” The surge line basically represents “stalling” of the airflow at the compressor inlet. As air passes through the air channels between the blades of the compressor impeller, boundary layers build up on the blade surfaces. These low momentum masses of air are considered a blockage and loss generators. When too small a volume flow and too high of an adverse pressure gradient occurs, the boundary layer can no longer adhere to the suction side of the blade. When the boundary layer separates from the blade, stall and reversed flow occurs. Stall will continue until a stable pressure ratio, by positive volumetric flow rate, is established. However, when the pressure builds up again, the cycle will repeat. This flow instability continues at a substantially fixed frequency, and the resulting behavior is known as “surging.”
The “choke line” represents the maximum centrifugal compressor volumetric flow rate as a function of the pressure ratio, which is limited for instance by the minimal cross-section of the channel between the blades, called the throat. When the flow rate at the compressor inlet or other throat location reaches sonic velocity, no further flow rate increase is possible and choking results. Both surge and choking of a compressor should be avoided.
In attempting to adapt and/or optimize available compressors for use on turbocharger assemblies suitable for various type internal combustion engines, rather than design totally new compressors, the problem most frequently encountered is that the available compressors have insufficient compressor map width, i.e., the operating range of the compressors is too narrow to satisfy the air requirements of the particular engine while, at the same time, operating efficiently under the speed (rpm) conditions imposed by the engine. In an attempt to design around this problem, engine manufacturers have been forced to offer narrower speed range engines than would otherwise be desirable. Alternatively, in some instances, where available and practical, greater capacity, albeit more expensive, compressors are employed.
The problem of the boundary layer separating from the blade can be reduced somewhat by using backward-swept blade tips. Blade backsweep results in long, gradually expanding airflow channels, which slows the airflow and produces less boundary layer separations. However, compressor efficiency is still limited by the flow instability.
An attempt to avoid surge can be found in U.S. Pat. No. 4,743,161 to Fisher et al. Fisher et al. show a recirculation passage in a turbocharger compressor housing. The recirculation passage is designed to produce a positive differential pressure on the inlet at choke and a negative differential pressure on the inlet at surge. While the recirculation passage helps to reduce the pressure differential, it creates additional problems. For example, the recirculation passage increases the amount of noise emitted, there are casting problems associated with creating a small recirculation passage inside the housing piece, there are increased manufacturing costs, and there are cleaning problems associated with keeping the recirculation passage clear from debris and preventing breakdown. Further, in recirculation, the same air is passed through the compressor passage twice, increasing the workload on the compressor.
Another approach involves a bypass port compressor. This turbocharger has a center channel that flows directly into the compressor wheel and also has an annular channel which acts as a bypass and provides flow either into or out of the compressor wheel. At low speeds, which might otherwise cause surge conditions because the volume of air provided is insufficient for the system's requirements, the bypass port allows additional air mass into the compressor impeller, allowing the system to reach equilibrium. At high speeds, which might otherwise cause a choke condition because the system's air requirements exceed the compressor's maximum flow rate, the port allows surplus air mass to be redirected from the compressor wheel.
However, the problem with this type of compressor is that there is a significant increase in noise. The port, or bypass, provides a direct path to the compressor wheel, and thus provides a means for the noise (and sound waves) generated by the high-speed revolutions of the compressor wheel to exit the compressor housing. Methods for controlling the emissions, such as inserting baffles along the annular channel, increase the cost of manufacture.
Another method for preventing surge conditions involves swirling the inducted airflow. When the induction volume falls to a level at which surging is apt to occur, it is known to swirl the inducted air flow upstream of the turbo compressor wheel in order to suppress or lower the surge limit of a turbo compressor. This reduces the angle of incidence of the incoming flow of air on the blades of the compressor wheel suppressing the surge limit. However, the problem with this approach is that turbulent flows created by pressure differentials cause a vibration, which under given operational conditions, tends to maximize or resonate to the degree of damaging the blades of the compressor. Moreover, construction of the vanes used to swirl the upstream air is complex and difficult to install in the confined space available in an induction housing.
Recently, WO 03/008787 to International Engine Intellectual Property Company, LLC was published disclosing an engine control unit (ECU) employed to reduce any significant turbocharger surge. The strategy for reducing surge conditions is implemented via a processor-based ECU. The ECU utilizes data relating to certain engine operating parameters to control the bleed of compressed charge air from the engine intake system via an exhaust valve located at the outlet of the compressor. The controlled bleeding counters any incipient surging of the compressor that results from increasingly retarding the timing of the exhaust valve opening/and the accompanying increase in fueling. By bleeding the air away from the intake manifold, the intake manifold pressure can increase without surge.
However, the problem with this invention is that it increases the cost of the turbocharger unit. Additionally, the useful benefits of the invention are countered by the expense of increased fuel consumption and reduced engine torque which occurs as a result of increased engine pumping loss.
The inventor saw a need for a device to reduce surge in a compressor impeller wheel. The device also needed to be cost efficient, fuel-efficient and inexpensive to manufacture.